The present invention relates generally to silicon carbide ceramic materials derived from silane polymers. More particularly, the present invention relates to new ceramic precursors that possess improved processing characteristics over generally similar, prior art, pre-ceramic polymers.
Considerable research is currently ongoing throughout the world on developing methods of producing silicon carbide ceramic fibers. Such fibers are unusually well suited for myriad potential applications where ceramic materials' characteristic properties, which include high strength, light weight and stability at high temperatures, are essential.
Since silicon carbide is brittle, and thus is generally unsuited to be formed into fibers, researchers have prepared silicon carbide fibers indirectly from polysilanes. For example, Baney et al., U.S. Pat. No. 4,310,651 ('651) discloses the synthesis of polysilane polymers, especially methylpolysilanes, that could be formed into fibers and then pyrolyzed, giving ceramic fibers. Unfortunately, these polymers are often difficult to process, having poor rheological characteristics and a tendency to burn spontaneously in air during spinning or other processing operations. Such difficulty in processing is a significant obstacle to using polysilane polymers as a route for the manufacture of silicon carbide fibers.
Work subsequent to Baney et al. '651 has sought to improve the processability of these polysilane polymers by modifying the basic polymer with various substituents. For example, Baney et al., U.S. Pat. No. 4,298,559, sought to make the polymers easier to handle by adding alkyl or aryl substituents via a grignard reaction. Seyferth et al., U.S. Pat. No. 4,639,501 discloses modifying another type of polysilane polymer via hydrosilylation reactions with organic or organosilicon compounds containing two or more alkenyl groups. Although the primary objective of Seyferth et al. appears to be to increase the yield of the final ceramic, another stated objective is to obtain a polymer that is stable at room temperature.
Although these prior efforts to improve polysilane polymers may improve certain characteristics of the polymer, they do not remedy what appears to be a major cause of the polymer's instability, i.e., the presence of a number of branching sites in the polymer backbone. The high degree of branching--which may lead to the formation of small, highly strained ring structures--seems to reduce the oxidative stability of the polymer, and probably also contributes to a rigidity in the polymer matrix that can result in poor rheology. Accordingly, it is desirable to chemically modify the polymer such that the polymer's oxidative instability relating to the branching sites and ring strain can be reduced without diluting the desirable characteristics of the silicon carbide end-product that make ceramic fibers so attractive.